Diene block polymers are materials that are useful for packaging, tires, shoe soles, adhesives and the like. Block polymers comprising at least one poly(vinyl aromatic) block and at least one polymerized diene block are referred to herein as diene block polymers. When the polymerized diene is polybutadiene the polymers are referred to herein as polybutadiene block polymers. Diene block polymers generally have non-elastomeric, poly(vinyl aromatic) end blocks and at least one elastomeric center block of polymerized conjugated diolefin, which diolefins are referred to herein as dienes. Properties of the diene block polymers depend on the quantity and composition of each block. When the polydiene block content comprises less than about half the weight of the block polymer, the copolymers are tough, non-elastomeric plastics. As the polydiene content increases, elastomeric qualities of the copolymer increase. Diene block polymers having more than about 50 percent by weight of polydiene blocks are considered thermoplastic elastomers. Such copolymers and their preparation are described in U.S. Pat. Nos. 3,251,905 to Zelinski; 3,265,765 to Holden et al.; 3,299,174 to Kuhre et al.; 3,231,635 to Holden et al.; 4,427,837 to Tung et al.; and Canadian Patent 50,466 to Milkovich. The teachings of each of which patents are incorporated herein by reference. These block polymers have physical properties much like other types of elastomers, yet they can be processed using processing equipment commonly used for thermoplastics. In other words, they can be injection molded, vacuum formed, blow molded, extruded, and the like.
Thermal qualities of diene block polymers have limited their industrial application. Polydiene in the copolymers generally leads to inherent instability of physical properties at elevated temperatures. Deterioration, that is, crosslinking, oxidation and degradation of physical properties, is observed when diene block polymers are heated sufficiently for common processing, such as molding and the like, especially when the heating is in the presence of oxygen. Yellowing of the copolymer is also frequently observed. Thus, for example, when diene block polymers such as those composed of styrene and butadiene are subjected to elevated processing temperatures, they often lose flexibility; and objects molded from them show surface crazing. The loss of flexibility is thought to result from crosslinking of the polydiene blocks. Many block polymers of poly(vinyl aromatics) and polydienes are formed in solution and must be heated to remove the solvent. Even this heating may promote crosslinking and deterioration. Deterioration observed on exposure to heat is referred to herein as thermal deterioration. Resistance to thermal deterioration is referred to herein as heat stability, or thermal stability. Thermal stability is often decreased by exposure to oxygen. Resistance to deterioration on exposure to oxygen is referred to herein as oxidative stability.
Triblock elastomeric copolymers with conjugated diolefin midsections and end blocks which incorporate alkylated vinyl aromatics have higher heat distortion temperatures than the corresponding copolymers with polystyrene end blocks. Processing such polymers, therefore, requires higher temperatures than does processing of the corresponding styrene block polymers. For instance, common processing temperatures for styrene block polymers are in the range of about 100.degree. C. to about 250.degree. C. Processing block polymers having alkyl vinyl aromatic monomers in their end blocks usually requires temperatures at least about 20.degree. C. higher than corresponding styrene block polymers. Processing at such temperatures causes substantial deterioration of unstabilized diene block polymers especially when they are also exposed to oxygen. Stabilizers and stabilizer systems known to these skilled in the art are not adequate to prevent substantial oxidation, crosslinking, surface and physical deterioration of alkylated vinyl aromatic block polymer exposed to such processing temperatures. Additional demands are placed on stabilizer systems when the copolymers are exposed to oxygen and heat in the preparation thereof. Such exposure is believed to start deterioration mechanisms that are continued and accelerated in subsequent processing.
A variety of stabilizers has been proposed for stabilizing diene block polymers. The use of tris(nonylphenyl) phosphite and 2,6-di-t-butyl-4-methylphenol in compositions of, optionally modified, block polymers having polystyrene and polybutadiene or polyisoprene blocks is disclosed in U.S. Pat. Nos. 4,120,915 and 4,292,414. Radial block polymers of butadiene and styrene have been stabilized with combinations of hindered phenols or alkylene bisphenols and substituted aromatic secondary amines, having at least two alkyl groups, at least one of which is ortho to the amine group, on each of two phenyl groups bonded to the amine nitrogen as disclosed in U.S. Pat. No. 3,629,372. Similar radial block polymers have also been stabilized with combinations of hindered phenols, organic phosphites and nitrogen-containing compounds as disclosed in U.S. Pat. No. 4,000,221. Deterioration physical properties of styrene-butadiene-styrene block polymers stabilized with such antioxidants as tetrakis[methylene(3,5-di-tert-butyl-4-hydroxyhydrocinnamate)]methane, dilaurylthiodipropionate, 2-(2'-hydroxy-5'-methyl phenyl)benzotriazole, 2-hydroxy-4-n-octoxybenzophenone, 1,2,3-tris(3,5-di-t-butyl-4-hydroxybenzyl)isocyanurate, and the like and mixtures thereof is illustrated, for instance, in "Stabilization of Thermoplastic Elastomers," by C. Mak, et al, in Rubber World, Oct., 1983, pp. 220-223. Generally, utility of these stabilizers is limited with respect to the type of polymer with which they are effective or with respect to the temperature range for which they are useful.
In view of the difficulties in stabilizing diene block polymers at temperatures desirably used in their processing, it would be useful to provide a stabilizing composition effective to stabilize diene block polymers against deterioration caused by exposure to elevated temperatures, especially those temperatures encountered in processing butadiene block polymers having alpha-methylstyrene/styrene copolymer blocks. Further, it would be useful to provide such a stabilizing composition that is effective even when the diene block polymers are exposed to oxygen and heat in recovering and processing of the copolymers.